Digital Radio Mondiale (DRM) is an open standard digital radio system designed for low frequency (148.5 KHz to 283.5 KHz), medium frequency (525 KHz to 1705 KHz) and high frequency (2.3 MHz to 27 MHz) signals, and may even be extended up to the 120 MHz range. The DRM system specification is described in European Telecommunication Standards Institute (ETSI) ES 201 980 Version 2.2.1 (2005-10), and which is incorporated herein by reference.
DRM signals are designed to fit within the existing amplitude modulation (AM) broadcast channels and spacing plan (based on 9 or 10 KHz spacing/bandwidth), however, the audio quality of DRM signals (near FM-quality) is significantly better than current AM radio system signals. Video, audio and other data may be transmitted within the DRM system. The current DRM specification provides for operation in accordance with various bandwidth modes, including 4.5, 5, 9, 10, 18 or 20 KHz, sometimes referred to as the “spectrum occupancy.” In addition, DRM signals may be broadcast simultaneously with AM signals in a simulcast mode.
The DRM system uses Orthogonal Frequency Division Multiplexing (OFDM), which includes the transmission of many narrow channels in parallel sub-carriers. Within the OFDM DRM system, various modulation schemes may be used, such as quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (16-QAM) and 64-QAM. The number of sub-carriers depends on the robustness mode chosen and the channel bandwidth within DRM. Currently, there are four robustness modes—A (e.g., 226 sub-carriers, 10 KHz bandwidth; 458 sub-carriers, 20 KHz bandwidth), B (206; 410 carriers), C (138; 280 carriers) and D (88; 178 carriers), which may be chosen depending propagation conditions. Bit rates ranging between 4.8 and 72 kb/s are achievable depending on the spectrum occupancy, robustness mode, modulation technique and coding rate.
Within DRM, the OFDM carrier signal constitutes the sum of a number of sub-carriers with baseband data on each sub-carrier being independently modulated using a modulation technique (e.g., QAM). The composite DRM OFDM signal is then used to modulate a main RF carrier. The benefits of using OFDM are known to those skilled in the art and, thus, need not be described further.
Spacing between the sub-carriers and the OFDM symbol length vary within DRM depending on the robustness mode. For example, the sub-carrier spacing is 41.66 Hertz in mode A and 46.88 Hertz in mode B. The OFDM symbol includes two portions—the useful portion and the guard interval. In mode A, the symbol length is 26.66 millisecond with a 24 millisecond useful portion and a 2.66 millisecond guard interval, while in mode B, the symbol length is also 26.66 millisecond but with a 21.33 millisecond useful portion and a 5.33 millisecond guard interval. The portion of the guard interval to the useful symbol's duration is different depending on the mode (A—1/9, B—1/4, C—4/11, D—11/14).
While some prior art techniques have proposed to demodulate the OFDM signal using discrete Fourier transforms (DFT), the fast Fourier transform (FFT) is typically preferred due to its reduced computational complexity. As a result, FFT processing (and inverse FFT (IFFT)) is utilized to construct and recover the OFDM composite signal in OFDM-based systems.
One specific prior art software-based DRM receiver, known as the “DREAM receiver,” receives the OFDM composite signal using an intermediate frequency (IF) of 12 KHz (down-converted from the conventional 455 KHZ IF of a standard heterodyne RF front-end) and samples the signal at 48 KHz. The DREAM receiver demodulates the received DRM OFDM signal using fast Fourier transform in the West (FFTW). This is a widely used free-software library that computes the DFT and its special cases. Though competitive with vendor-optimized programs, the FFTW is not tuned to a fixed machine. Rather, FFTW uses a planner to adapt its algorithms to the hardware platform in order to increase performance.
In the PC-based DREAM receiver, the 48 KHz sampled signal is input to the FFTW blocks after determining the FFT window. All DRM robustness and spectrum occupancy modes may be sampled at this frequency without adjustments, and the DFT's point number (i.e., number of samples of the useful part of the symbol) is as follows: A mode—1152; B mode—1024, C mode—704; D mode—448. Four major algorithms are implemented in FFTW, including Cooley-Tukey, Prime-factor, Rader and Bluestein. The FFTW planner measures actual run time of different plans and selects the fastest one. FFTW may also provide a mode of operation where it quickly returns a “reasonable” plan, but this may not be the fastest. For the PC-based DREAM receiver, the FFTW may be a reasonable implementation for DRM OFDM demodulation.
When using FFT, it is generally desirable to sample the desired signal using a number of samples equal to a power of two (sometimes referred to as “points of power of 2”). However, because of the symbol guard interval length to useful symbol portion (as described above) specified in the DRM specification (see above), none of the robustness modes (except B) can achieve simultaneously points of power of 2 in the useful symbol portion and integer points in the guard interval. Therefore, FFTW as used in the DREAM receiver results in higher complexity than FFT.
Accordingly, there is needed a new DRM receiver and method that uses an FFT for demodulating DRM OFDM signals in order to reduce computational complexity. Such FFT may be implementable in hardware, such as a field programmable gate array (FPGA), application specific integrated circuit (ASIC), or digital or other signal processor (programmed accordingly).
As noted above, the DRM specification contemplates various simulcast modes because of the relative time and cost necessary to shift the current AM spectrum to the DRM-encoded digital signals. Simulcast transmission of DRM and AM signals may be performed by the juxtaposition of an analog AM channel with a DRM digital channel (e.g., channel spacing of 10 KHz between the AM carrier frequency and the digital reference frequency). Therefore, both analog (AM) and digital (DRM) signals will likely coexist for some time. In a DRM receiver capable of receiving both DRM encoded and traditional analog AM signals, a separation filter is required. Due to the proximity of the frequency bands, it is difficult and complex to design the separation filter. For example, in robustness mode A and spectrum occupancy mode 0, a filter with pass-band of 2083.33 Hertz (signals on sub-carriers #2 to #102) and transient-band 83.33 Hertz would be required (the interval between the digital signal and analog signal is 83.33 Hertz).
Using a conventional MATLAB filter design tool, and assuming desired filter parameters of 1 dB in-band ripple and 40 dB stop-band attenuation, when the sampling frequency is relatively high (100 KHz or greater), the order of the filter is in the thousands.
Accordingly, there is needed a new DRM receiver and method including a digital and analog separation filter to receive and separate conventional analog AM signals and DRM-encoded digital signals that are transmitted in close proximity within the transmission band.